Seismology is used for exploration, archaeological studies, and engineering projects that require geological information. Exploration seismology provides data that, when used in conjunction with other available geophysical, borehole, and geological data, can provide information about the structure and distribution of rock types and their contents. Such information greatly aids searches for water, geothermal reservoirs, and mineral deposits such as hydrocarbon reservoirs and/or veins. Most oil companies rely on exploration seismology to select sites in which to drill exploratory oil wells.
Traditional seismology employs artificially generated seismic waves to map subsurface structures. The seismic waves propagate from a source down into the earth and reflect from boundaries between subsurface structures. Surface receivers and/or sub-surface receivers detect and record direct or reflected seismic waves for later analysis. Though some large-scale structures can often be perceived from a direct examination of the recorded signals, generally the recorded signals are processed to remove distortion and reveal finer detail in the subsurface image.
When sedimentation and tectonic processes produce dip and layer thickness variations in anisotropic media, their velocity structures may be approximated as Tilted Transversely Isotropic (TTI) that induces a directional dependence on wave propagation. For example, in thrust belts such as the Canadian foothills reservoirs, thick sequences of dipping sandstone and shale layers generate a tilted symmetry axis. Also, in some salt domes (e.g., in the Gulf of Mexico area), the strata around the salt flank are tilted by the movement of the salt. Accurate analysis of seismic data for anisotropic media is not a trivial task.
One example seismology technique, known as walk-away vertical seismic profile (VSP) survey, determines the response of receivers in a borehole to excitation by at least one seismic source located at various distances from the well-bore. However, the results are often affected by seismic anisotropy. The determination of anisotropic parameters from surface seismic data is often difficult, due to relatively poor data quality and the relatively low frequencies at which the measurements are made.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description are not intended to limit the disclosed embodiments to the particular forms shown, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the appended claims.